The demand for the use of alternative energy or clean energy has been increasing due to a rapid increase in the use of fossil fuels, and the power generation and accumulation fields using electrochemical reactions are the currently most actively researched fields as a part thereof.
A secondary battery is a current representative example of electrochemical devices using electrochemical energy. Also, an application area thereof has gradually been increasing. Recently, as technical development and the demand for portable devices such as a portable computer, a cellular phone, a camera, and the like have been increasing, the demand for a secondary battery as an energy source has been rapidly increasing. Much research for a lithium secondary battery that provides high energy density and operation potential and has a long cycle lifespan and a low self-discharge rate among secondary batteries has been performed such that secondary batteries have been commercialized and generally used.
Generally, a secondary battery includes a positive electrode, a negative electrode, and an electrolyte. Here, since lithium ions which come out of a positive electrode active material serve to transfer energy while shuttling between both of the electrodes by being intercalated into a negative electrode active material such as carbon particles in a first charge and being deintercalated therefrom in case of discharge, the secondary battery may be charged and discharged.
For example, a lithium secondary battery has a structure in which a lithium electrolyte is impregnated in an electrode assembly which includes a positive electrode including a lithium transition metal oxide as an electrode active material, a negative electrode including a carbon-based active material, and a porous separator. The positive electrode is manufactured by coating an aluminum foil with a positive electrode mixture including the lithium transition metal oxide, and the negative electrode is manufactured by coating a copper foil with a negative electrode mixture including the carbon-based active material.
Among silicon-based materials which have recently been researched as high-capacity materials, a silicon-carbon composite active material is an active material formed by disposing a silicon-based material on a surface of a carbon-based negative electrode active material and has excellent discharge efficiency (80%) with a higher capacity than a theoretical capacity (372 mAh/g) of carbon-based negative electrode active materials such that it is expected to gain popularity as a high-capacity secondary battery material.
However, due to weak binding forces between interfaces of carbon and silicon, a phenomenon in which an outermost silicon layer falls off of the carbon occurs during an electrode manufacturing process. Since the phenomenon reduces conductivity of the silicon and increases an irreversible capacity thereof, a capacity and efficiency thereof are reduced.
Accordingly, it is necessary to develop a negative electrode active material capable of providing excellent conductivity while increasing binding forces between carbon and silicon.